The current invention is a process for recovering and reusing the active materials used in electrodes of electrochemical cells. In particular, the invention is a process and device for recovering the active materials deposited on flexible sheet electrode substrates. The process is directed primarily at recovering active material from scrap and trimmed electrode material resulting from the manufacture of cells.
Many secondary electrochemical cells are fabricated from two or more relatively flexible sheet electrodes separated by a nonconductive layer and wound in a spiral fashion into what is commonly known as a "jelly roll" configuration. They are then inserted into a container and surrounded by an electrolyte. Attaching electrical terminal connections to the partnered electrodes completes the cell. The individual electrodes are fabricated by depositing an electrochemically active material in a thin layer onto a current collector substrate. Example methods for completing this process for nickel-cadmium and nickel-metal hydride cells are provided in the U.S. Pat. Nos. to Rampel et al. (5,064,735), Pensabene et al. (5,466,546) and Kinoshita el al. (5,527,638). Typically, the active material is initially in a powdered form that is combined with solvents and other materials to form a paste. This paste is physically pressed or extruded onto the collector substrate. Other materials forming the paste include organic binders such as polyacrylates and copolymers added to improve adhesion. Both aqueous and nonaqueous binders are used in various electrode systems. Methods of forming electrodes using copolymer binders are provided in the patent to Kinoshita el al. Carbon or other additives may also be included to improve conduction or other electrochemical properties. In order to form the "jelly roll" configuration, the collector substrate is typically a thin flexible conductive sheet such as nickel plated steel that allows the significant bending required. Sheet electrodes often include a pattern of perforations over the effective area. Considerable effort has been expended in the technology to develop and maintain processes to deposit the active materials securely onto this flexible conductive sheet. During the winding operation, when the electrodes are rolled and flexed, the active material is subjected to impacts and strains that may disrupt it from the conductive sheet. In addition to maintaining physical attachment of the active material to the conductive substrate, electrical continuity must also be maintained. Consequently, electrodes are processed in a manner in which the active material is very fixedly attached to the sides of the conductive substrate. This is accomplished in part by using specific binders that hold the active material together and which in effect "glue" the active material in place onto the substrate. These binders are intermixed with the active material together with a solvent to form the active material paste. Deposition of the active material onto the electrode substrate is carried out in a variety of methods including extrusion and roll-coating. After this paste or slurry is deposited on the conductive sheet, the solvent is removed by drying. The solid binder remains to bind the active material tightly to the conductive sheet. At this stage the electrode may be pressed or calendered to compress the active material onto the substrate, increasing density and reducing the total thickness of the electrode. This calendering may also increase the adhesion of the active material and binder onto the substrate.
During fabrication of such electrochemical cells, scrap electrode material is often a byproduct. This may be in the form of strip scrap cut from continuous stock electrode to accomplish proper sizing of the finished electrode. Another source of scrap is electrodes damaged during processing or in some way not meeting quality or manufacturing standards. This scrap electrode material contains active material that in accumulation has considerable value but is typically discarded or lost when the scrap is sold or otherwise disposed of. To reduce the losses from scrapped active material, attempts have been made to recover and reuse the active material from scrap electrode stock and recovered used cells. Attempts to recover the active material are hindered by the very fact that the active material is intentionally affixed so well to the conductive substrate. The first step in recovering scrap active material is separation of the active material from the supporting electrode substrate. Due to the tenacious manner in which the active material is deposited, application of solvents alone is often insufficient to separate the active material from the substrate.
Several methods are known for recovery and reuse of specific active materials in some secondary cell processes. However, these focus on separation of the active material from the binder and other matrix constituents to bring the active material back to virgin condition. The separation processes used in these methods typically include chemical separation steps in which the binders are effectively washed out or otherwise lost. The recovered active material is then reprocessed at some point to reintroduce new binder material before it is deposited anew onto a current collector. The present methods have an added benefit in reduced chemical wastes reducing environmental impact. Because neither the binders, nor solvents used to dissolve binders, are removed from the process system, the problem of their disposal is eliminated.
In order to most efficiently reuse scrap active material, a process is needed for reclaiming active material from an electrode together with binders and other constituents and reusing this product directly in electrode fabrication. In this way, a greater proportion of the scrap may be saved and reused with a minimum of energy and material expenditure and environmental impact.